[Python] Pydantic Validation Issue with Union Models and extra = "forbid"

I am working with Pydantic models in FastAPI, and I have a structure where RCSMessage can be one of several possible models. I am using Union to allow multiple message types and extra = "forbid" to prevent unknown properties. However, when I send a JSON with a property that does not exist in any model, Pydantic does not raise an error as expected.

Pydantic Models


from pydantic import BaseModel, Field
from typing import Optional, Union
from datetime import datetime

class MessageContact(BaseModel):
userContact: Optional[str] = Field(
None, pattern=r"^\+\d{1,15}$"
) # MSISDN E.164 format
chatId: Optional[str] = None # Example UUID

class RCSMessageBase(BaseModel):
msgId: Optional[str] = None
status: Optional[str] = None
trafficType: Optional[str] = None
expiry: Optional[datetime] = None # ISO 8601 date-time format
timestamp: Optional[datetime] = None # ISO 8601 date-time format

class Config:
extra = "forbid"

class SuggestedResponse(BaseModel):
suggestedResponse: Optional[dict] = None

class Config:
extra = "forbid"

class SharedData(BaseModel):
sharedData: Optional[dict] = None

class Config:
extra = "forbid"

class IsTyping(BaseModel):
isTyping: Optional[str] = Field(None, pattern="^(active|idle)$")

class Config:
extra = "forbid"

RCSMessageType = Union[
SuggestedResponse, SharedData, IsTyping
]

RCSMessage = Union[RCSMessageBase, RCSMessageType]

class RCSMessageWithContactInfo(BaseModel):
RCSMessage: RCSMessage
messageContact: MessageContact

Issue


When I send the following request:

{
"RCSMessage": {
"isTypingNoExiste": "active"
},
"messageContact": {
"userContact": "+14251234567"
}
}


I expect Pydantic to raise an error because isTypingNoExiste is not a valid property in IsTyping. However, the validation does not fail, and the JSON is accepted.

Question


How can I make Pydantic reject any unknown properties within RCSMessage, ensuring that only the defined properties in the models within the Union are allowed?

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